Biodiesel Purification Options

By Al Schultz | March 23, 2010

The base-catalyzed reaction of triglyceride with methanol produces fatty acid methyl esters. It is not until this material is purified to meet ASTM/EN specification that it should be considered biodiesel. Many purification techniques exist for biodiesel manufacturers, including methods to clean up feedstocks, crude biodiesel, and most recently, to polish biodiesel for improved cold temperature performance.

Fats and oils can be treated by a variety of methods to convert them into raw materials for the biodiesel industry. While purification for biodiesel can be done at various steps within the process, up front feedstock purification pays the largest dividends since the overall process can be improved dramatically. It is preferable to remove potentially problematic materials from the feedstock prior to processing this feedstock into biodiesel. Generally speaking, the steps of pretreatment can include either chemical or physical refining. Physical refining includes extraction, degumming, bleaching and deodorizing, not all of which are necessary to convert oil into useable feedstock for the biodiesel industry. For materials with high levels of impurities, physical treatment is the preferred method of purification. In cases where biodiesel manufacturers are looking to remove minor amounts of impurities, chemical refining is the preferred method. The oil can be passed through a column of the appropriate ion-exchange resin, or bleaching earth, for example, to remove the last trace amounts of impurities. Many oil and biodiesel suppliers have their own proprietary techniques for physical refining of oil.

Once the transesterification reaction is completed and the glycerol is separated via gravity separation from the methyl ester, this crude product must be further processed to remove impurities in order to meet the appropriate specifications. Impurities removed during this step include residual quantities of catalyst, glycerol and soaps. The favored methods in the biodiesel industry include water wash and nonwater wash. Water wash, as the name implies, relies on simple water addition, followed by separation, to remove the residual water soluble species. The water used in this step can be recycled back into other points of the biodiesel manufacturing process. For nonwater washing, two materials have found excellent commercial success: ion-exchange resins and magnesium silicates (or other inorganic adsorbents such as bleaching earth). These materials work by chemically adsorbing the impurities, thereby eliminating them from the biodiesel stream. The use of inorganic materials typically involves addition of the solid to a stirring solution of biodiesel. Slurry is made, and after stirring this solution for the requisite amount of time, separation with a drum filter or related filtration techniques provides the biodiesel in purified form. To use the ion-exchange resins, producers typically install a column filled with the dry resin, and the biodiesel is passed through this resin in continuous flow until breakthrough occurs. Some reports indicate as much as 2,000 liters of biodiesel can be processed through only one liter of resin or adsorbent.

In recent times, more stringent cold temperature performance standards have been imposed onto the biodiesel industry. The pioneering work published in 2006 points to monoglycerides, sterol glucosides, soap and water as the culprits causing poor cold temperature performance. Suppliers of biodiesel have a few options to overcome these cold temperature performance issues. One such method-cold filtration-involves cooling the biodiesel and filtering it while it is still cold. This method removes impurities that have poor solubility at these reduced temperatures. Another potential method of purification is an energy-intense process that involves distillation of the final biodiesel product. Both of these methods can involve the purchase of some elaborate processing equipment.

To help resolve this problem for the biodiesel industry, researchers at The Dow Chemical Company have recently developed resin technology specifically targeted toward the removal of impurities such as monoglycerides and sterol glucosides. The challenge for producing such a resin lies in selectivity, the preferential removal of small quantities of monoglycerides and sterol glucosides in the sea of methyl ester. To overcome this problem, a material with the correct hydrophile-lipophile balance, as well as the proper pore structure to effectively adsorb these polyhydroxylated materials, was developed. Reports indicate that the use of these resins causes a reduction of monoglycerides by 80 to 90 percent. Since the removal of these impurities relies on an adsorption mechanism, the resin is regenerable, creating a cost-effective solution to the cold temperature performance issues the biodiesel industry is currently facing.

Al Schultz is a senior scientist for Dow Water & Process Solutions. He can be reached at alfredkschultz@dow.com.

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